joel, for the crabs I recommend a long screwdriver and a bit of patience. You'll find that you can catch them out in the open pretty regularly or spear them lurking in a hole. It's pretty funny to watch the starfish and hermit crabs come running to eat what's left.
For the mantis, you may just have to try Steven's trick. I haven't had any luck with the trap either so I'm hoping as time goes by it gets more comfortable in the tank and I can catch it out in the open, track it to a hole, and then use Steven's trick to get it out.
For the mantis, you may just have to try Steven's trick. I haven't had any luck with the trap either so I'm hoping as time goes by it gets more comfortable in the tank and I can catch it out in the open, track it to a hole, and then use Steven's trick to get it out.